Center beam rail road cars have a pair of end structures mounted on railroad car trucks. A center sill extends the length of the car between the end structures. A deck extends laterally outward from the center sill above, and between, the end structures. A pair of end bulkheads stand at the ends of the car and extend transversely to the rolling direction of the car. A center beam structure, typically in the nature of a truss, stands upright from the deck and runs along the longitudinal centerline of the car between the end bulkheads. The center beam is a deep girder beam whose bottom flange is the center sill, and whose top flange is the top truss (or analogous structure) of the car. Typically, a web work structure for carrying vertical shear loads, such as an open framework of posts and diagonal braces, extends between the center sill and the top truss. An upper beam assembly, that is, the upper or top flange end of the center beam, is usually manufactured as a wide flange, or flange-simulating truss, both to co-operate with the center sill to resist vertical bending, and also to resist bending due to horizontal loading of the car while travelling on a curve. Center beam cars are commonly used to transport packaged bundles of lumber, although other loads such as pipe, steel, engineered wood products, or other goods can also be carried.
The space above the deck on each side of the center beam forms a bunk upon which bundles of wood can be loaded. The base of the bunk has risers that are mounted to slant inward, and the center beam itself is tapered from bottom to top, such that when the bundles are stacked, the overall stack leans inward toward the longitudinal centerline of the car. The load is most typically secured in place using straps or cables. The straps extend from a winch device at deck level, upward outside the bundles, to a top fitting. The top fitting can be located at one of several intermediate heights for partially loaded cars. Most typically the cars are fully loaded and the strap terminates at a fitting mounted to the outboard portion of the upper beam assembly. Inasmuch as the upper beam assembly is narrower than the bundles, when the strap is drawn taut by tightening the pawl, it binds on the upper outer corner of the topmost bundle and exerts a force inwardly and downwardly, tending thereby to hold the stack in place tight against the web of the center beam.
Each bundle typically contains a number of pieces of lumber, commonly 2×4, 2×6, 2×8 or other standard size. The lengths of the bundles vary, typically ranging from 8′ to 24′, in 2′ increments. The most common bundle size is nominally 32 inches deep by 49 inches wide, although 24 inch deep bundles are also used, and 16 inch deep bundles can be used, although these latter are generally less common. A 32 inch nominal bundle may contain stacks of 21 boards, each 1½ inch thick, making 31½ inches, and may include a further 1½ inches of dunnage for a total of 33 inches. The bundles are loaded such that the longitudinal axes of the boards are parallel to the longitudinal, or rolling, axis of the car generally. The bundles are often wrapped in a plastic sheeting to provide some protection from rain and snow, and also to discourage embedment of abrasive materials such as sand, in the boards. The bundles are stacked on the car bunks with the dunnage located between the bundles such that a fork-lift can be used for loading and unloading.
It has been observed that when the straps are tightened, the innermost, uppermost boards of the topmost bundle bear the greatest portion of the lateral reaction force against the center beam due to the tension in the straps or cables. It has also been observed that when these bundles bear against the vertical posts of the center beam, the force is borne over only a small area. As the car travels it is subject to vibration and longitudinal inertia loads. Consequently the plastic sheeting may tend to be tom or damaged in the vicinity of the vertical posts, and the innermost, uppermost boards can be damaged.
The physical damage to these boards may tend to make them less readily saleable. Further, whether or not the boards are damaged, if the plastic is ripped, moisture can collect inside the sheeting. This may lead to the growth of molds, and may cause discolouration of the boards. In some markets the aesthetic appearance of the wood is critical to its saleability, and it would be advantageous to avoid this discolouration.
In part, the difficulty arises because the bearing area may be too small. Further, the join between the upstanding web portion of the center beam and the upper beam assembly can coincide with the height of the topmost boards. This join is not always smooth. Further still, when the posts are fabricated, the flanges of the posts may not stand perfectly perpendicular to the webs of the respective posts. That is, the post flanges may not be co-planar with the side webs, or legs, of the adjoining top chord, such that one edge of the flange may be twisted so that it bears harder against the bundles than another.
It is also desirable that the bundles stack squarely one upon another. Although it is possible to use wooden battens at the top end of the center beam, this will tend to cause the top bundle to sit outwardly of its neighbours. It has been observed that a thin wooden batten, of ¾″ thickness may tend to bow inwardly between adjacent posts, and may not spread the wear load as much as may be desired. A 1½ inch thick wooden batten may have a greater ability to resist this bowing effect. However, the space available for employing a batten may tend to be limited by the design envelope of the car. Inasmuch as is advantageous to load the car as fully as possible, and given that the design of the car may usually reflect a desire to maximize loading within the permissible operational envelope according to the applicable AAR standard, the use of a relatively thick wooden batten may tend to push the outside edge of the top bundle outside the permissible operational envelope. Wooden battens may also be prone to rotting if subject to excessive exposure to moisture, or may be consumable wear items that may require relatively frequent periodic replacement.
It would be desirable to have an upper beam assembly that is integrated into the structure, that is formed to spread the bearing load across a larger area, that would tend to resist the bowing phenomenon, that would tend not to require frequent replacement, and that would tend not to be prone to rotting.